In recent years, development of stereoscopic image display apparatuses (autostereoscopic image display apparatuses) without glasses has been promoted. Many of them use an ordinary plane display device. Angles of light rays illuminated from the plane display device are controlled by placing some optical plate in front of or behind a display plane of the plane display device. When viewed from a viewer, therefore, light rays look as if they are illuminated from objects located at several cm away from the plane display device in front of or behind it due to binocular parallax.
It is said that in the autostereoscopic image display apparatus the resolution of a three-dimensional image, the depth projection reproduction capability thereof, and the viewing angle thereof are in trade-off relations. For example, it is now supposed that a liquid crystal display device (hereafter referred to as LCD as well) which displays an elemental image is disposed behind a slit or a lens which controls light rays as an optical plate included in the autostereoscopic image display apparatus. If in this case the number of parallaxes is determined and the viewing angle serving as a range in which a parallax image is distributed is narrowed, then the light ray density becomes high and the depth projection reproduction capability increases, but the range in which the stereoscopic image is visible normally becomes narrow. If the number of parallaxes is determined and the viewing angle is made wider, then the depth projection reproduction capability decreases, but the range in which the stereoscopic image is visible normally becomes wide.
Furthermore, development of an autostereoscopic image display apparatus having a changeover function between two-dimensional image display and three-dimensional image display is conducted. In this autostereoscopic image display apparatus, a lenticular lens is not used as the optical plate, but, for example, a gradient index lens having a structure obtained by filling a space between two parallel transparent substrates with liquid crystal and providing inclination of directors of liquid crystal with distribution by means of voltage control of the liquid crystal is used as the optical plate. This gradient index lens becomes a liquid crystal GRIN lens (gradient index lens) based on refractive index distribution depending upon the place generated when light rays having a determinate polarization direction is incident on two parallel transparent substrates. Since this GRIN lens can be switched to ON and OFF by changing a voltage, it can be utilized to change over between two-dimensional image display and three-dimensional image display.
However, use of the gradient index lens as the optical plate in the autostereoscopic image display apparatus having the changeover function between two-dimensional image display and three-dimensional image display poses a problem that display degradation is caused by disposition of spacers used in this gradient index lens.